Fine particulate matter in the air from cars and trucks, cooking, fires, and industry can give you cancer, heart attacks, and shorten your lifespan. Poor air quality can have a number of other bad effects. You can make a DIY air quality monitor (AQM) for about $20 that can warn you of poor air quality. You can add internet datalogging for another few bucks. This will give you a rough measurement of the air quality of your environment, and can help you determine if you should do anything about it (get an air filter, etc).
Tiny particles in the air, generated from cars, trucks, powerplants, industry, and even cooking and burning fires at home, are causing cancer, heart attacks, and premature death among other ailments.
The risk of getting lung and brochus cancer stand at around 7% today, though around 80-90% of this is thought to be due to smoking.